Investigation of Thermal Stability and Continuous CO2 Capture from

Article pubs.acs.org/IECR

Investigation of Thermal Stability and Continuous CO2 Capture from Flue Gases with Supported Amine Sorbent Wenying Zhao, Zhi Zhang, Zhenshan Li,* and Ningsheng Cai Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization & Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Amine-functionalized material has gained attention as a promising solid sorbent for CO2 capture. To obtain a high concentration of CO2 for storage, the thermal regeneration temperature of supported amine sorbents should be higher than 135 °C under pure CO2. The thermal stability of solid amine sorbents will become a problem of significance. This study investigated the thermal stability of supported amine sorbents at 140 °C from the perspective of amine types and reactors, with a focus on how to improve the thermal stability of solid amine sorbents. Supported amine sorbents were prepared by impregnating polyethylenimine (PEI) and tetraethylenepentamine (TEPA) of different molecular weights on silica particles. The thermal stability of these amine-functionalized silica particles was investigated as a function of temperature and molecular weight in a thermogravimetric analyzer (TGA) and in a single-fluidized-bed reactor. An important finding was that the supported amine sorbents exhibited much higher thermal stabilities in a single-fluidized-bed reactor than in a TGA for long-term operation (100 h) at 140 °C, because the evaporation of the amines could be greatly inhibited in the fluidized-bed reactor because of the presence of amine vapor in the gas phase. Based on this finding, a simple and effective method of inhibiting amine evaporation was proposed and demonstrated. Furthermore, continuous CO2 capture was demonstrated using PEI-impregnated silica particles in a laboratory-scale dual-fluidized-bed reactor (DFBR). The CO2 capture efficiency was approximately 80% in the absorber during 2 h of operation, and the sorbent still maintained good stability and performance.

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INTRODUCTION It is now well accepted that the release of CO2 from fossil-fuel combustion contributes to the enhanced greenhouse effect with possible disastrous effects on Earth’s climate.1,2 An approach to reduce CO2 release is CO2 capture and storage (CCS). The estimated costs for CO2 transportation and storage are much smaller thanthe cost of CO2 capture.2,3 Therefore, reducing the cost of CO2 capture is necessary to make CCS more economically attractive. Aqueous amine scrubbing is the stateof-art technology for CO2 capture, but it is considered to be energy-intensive and expensive for large-scale CO2 separation, in addition to being corrosive and toxic in nature.4,5 To overcome these shortcomings, various amine functional groups supported on solid porous substrates by wet impregnation or covalent tethering or in situ polymerization within the pores have gained attention in recent years.6−18 For amine-functionalized sorbents, the adsorption capacity of CO2 is very important, and most researchers have been focusing on improving the adsorption capacity by applying various amine compounds to mesoporous supports under different conditions.12,19−21 Yan et al.19 and Sharma et al.22 obtained a CO2 adsorption capacity of 2.4 mmol·g−1 by impregnating polyethylenimine (PEI) on SBA-15 and MCM-41, respectively. Xu et al.23 compared the effect of PEI loading on MCM-41 and obtained a maximum capacity of 3.02 mmol·g−1 with a PEI loading of 75 wt % in a pure CO2 atmosphere at 75 °C. Song et al.21 investigated a series of mesoporous silica materials impregnated with PEI, including MCM-41, MCM-48, SBA15, SBA-16, and KIT-6, to evaluate the CO2 adsorption− desorption behaviors in a thermogravimetric analyzer (TGA), achieving a maximum adsorption capacity of 3.07 mmol·g−1. Qi et al.24 developed a nanocomposite sorbent based on © 2013 American Chemical Society

mesoporous silica capsules functionalized with oligomeric amines [PEI, tetraethylenepentamine (TEPA)], obtaining a highest adsorption capacity of 7.9 mmol·g−1 under a simulated humid flue gas at 75 °C. In the published literature, mesoporous silica molecular sieves such as MCM-41,7 SBA15,19 and SBA-1625 have mainly been utilized as supports for the preparation of amine-containing sorbents because of their high surface areas, large pore volumes, and uniform pore structures. However, these substrates account for more than 90% of the absolute sorbent preparation cost26−28 and are commercially unavailable. Therefore, commercial and costeffective porous materials should be chosen to reduce the cost of supported amine sorbents. The energy consumption for CO2 capture using supported amines decreases with increasing sorbent working capacity. When the working capacity reaches 2−3 mmol·g−1, the energy requirement of CO2 separation processes is reduced by 30− 50% compared with that of an amine solvent system.15 The energy requirement decreases slightly with working capacity at values above 2.5 mmol·g−1, whereas the cost of sorbent preparation rises sharply. From the point of view of regeneration costs, it is ideal for solid sorbents to reach av working capacity of 2−3 mmol·g−1. It has been pointed out that the deactivation, kinetics, and novel reactors are key issues that must be better studied for amine-functionalized sorbents.29 A crucial issue for amine-functionalized sorbents is their thermal Received: Revised: Accepted: Published: 2084

November 25, 2012 January 6, 2013 January 9, 2013 January 9, 2013 dx.doi.org/10.1021/ie303254m | Ind. Eng. Chem. Res. 2013, 52, 2084−2093

Industrial & Engineering Chemistry Research

Article

in 40 mL of methanol under stirring for about 15 min, after which 10 g of dried Q-10 was added to the TEPA/methanol or PEI/methanol solution. The slurry was continuously stirred for 2 h and then dried at 100 °C for 10 h. The synthesized samples are denoted as PEI600/Q-10, PEI1200/Q-10, PEI1800/Q-10, and TEPA/Q-10 depending on the amine used in the synthesis. Thermal Stability Experiments in a TGA and in a Single Fluidized Bed. The thermal stabilities of four types of supported amine sorbents were studied first in a TGA. Approximately 10 mg of supported amine was placed in a platinum crucible and then heated at 100 °C and kept for 30 min in a N2 flow of 100 mL·min−1 to remove the moisture in the sorbent. Then, the temperature was increased to 400 °C at a rate of 5 °C·min−1 with the same flow rate of N2 to monitor the weight change with temperature. Cyclic adsorption capacity was also studied in a TGA. The adsorption was carried out at 60 °C and 10 vol % CO2/N2 at a total flow rate of 100 mL·min−1 for 10 min, and the regeneration was carried out at 105 °C under pure N2 at a total flow rate of 100 mL·min−1 for 10 min. The mass loss and adsorption capacity were analyzed and compared after each cycle. For the four types of sorbents, the thermal stability was investigated at 140 °C for 5 h under pure N2 at a total flow rate of 100 mL·min−1. The loss of weight was ascribed to the evaporation of amine due to the weak van der Waals force between the amine molecules and the silica surface. A bubbling fluidized-bed reactor was employed for the thermal stability investigation. The dual-layer quartz reactor with an inner diameter of 30 mm is displayed in Figure 1. The

and chemical stability during the regeneration process. The thermal instability stems from amine evaporation, whereas the chemical instability comes from amine degradation induced by CO230−34 and other impurities such as O2,35,36 SO2,37 and NOx.38−41 The stability of amine-modified sorbents is a significant property that determines the lifetime and replacement frequency of the sorbent and greatly affects the economics. Drage et al.30 studied the stability of silicaimmobilized PEI in air, nitrogen, and CO2 and pointed out that a good cyclic regeneration capacity (2.0 mmol·g−1) was obtained by temperature-swing adsorption, although there was a loss of cyclic capacity and lifetime of the sorbent because of the secondary reaction between amine and CO2 to form an irreversible urea linkage above 135 °C. Several CO2-induced degradation mechanisms were proposed by Wu et al.31 and Sayari and co-workers.32,33 Sayari and Belmabkhout34 reported that water vapor improved the cyclic stability of amineimpregnated sorbent greatly at a regeneration temperature of 105 °C. However, the thermal stability of amine-functionalized sorbents has still not been investigated sufficiently, especially at higher temperatures (>135 °C). At the same time, to date, almost all investigations have been conducted in a thermogravimetric analyzer (TGA) or a fixed bed. Nonetheless, a fluidized-bed reactor is potentially an appropriate option for CO2 capture in a practical process because of the huge amount of flue gas, fast reaction kinetics, and high adsorption capacity of amine-containing sorbents. In a practical fluidized-bed reactor, the conditions are very different from those in a TGA in many aspects such as fluid dynamics, heat transfer, and atmosphere. Veneman et al.42 investigated continuous CO2 capture by a circulating fluidized-bed reactor using a supported amine sorbent and reported that approximately 56% of the CO2 introduced was captured from simulated dry flue gas during 2 h of operation. However, there is still a lack of sufficient information about adsorption− desorption processes and the stability of amine supported sorbents in fluidized-bed reactors. In this work, the thermal stabilities and cyclic adsorption capacities of impregnated low-cost silica gel with amines of different molecular weights were investigated in detail in a TGA. The thermal stability in a single-fluidized-bed reactor was compared with the results in the TGA under harsh conditions. Finally, a dual-fluidized-bed system was designed and built to demonstrate long-term continuous CO2 capture. Post-test analysis was also undertaken in a TGA for the sorbent after dual-fluidized-bed experiments to study the sorbent stability.

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EXPERIMENTAL SECTION Sorbent Preparation. Commercial low-cost silica gel with a mean pore diameter of 10 nm (CARIACT Q-10), a Brunauer−Emmett−Teller (BET) surface area of 273 m2·g−1, and a pore volume of 1.23 cm3·g−1 was supplied by Fuji Silica Chemical Ltd. and used as the support material impregnated by tetraethylenepentamine (TEPA) and polyethylenimine (PEI). TEPA, supplied by XiLong Chemical Ltd., has a molecular weight of 189.3 g·mol−1 and contains three primary amine groups and two secondary amine groups, giving a total amine content of 26.4 mmol·g−1. Branched PEI (Aldrich, average Mn ≈ 600, 1200, 1800) contains 30% primary amine and 40% secondary amine, with a total amine content of 23.2 mmol·g−1.43 TEPA- or PEI-functionalized Q-10 silica gel was prepared by wet impregnation according to the reported procedure.23 In each case, 5 mL of TEPA or PEI was dissolved

Figure 1. Schematic diagram of a single-fluidized-bed reactor system.

space between the dual tubes was 10 mm. As seen in Figure 1, the reactor had two stages with heights of 200 and 278 mm for the lower and upper stages, respectively. About 30 g of TEPA/ Q-10 with a particle size of 0.15−0.3 mm was placed into the upper stage. Approximately 10 mL of liquid TEPA was placed in the lower stage to investigate the effect of amine vapor on the thermal stability. Circulating hot oil was circulated through the space between the tubes to control the reactor temperature. 2085

dx.doi.org/10.1021/ie303254m | Ind. Eng. Chem. Res. 2013, 52, 2084−2093


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Figure 2. Schematic diagram of a dual-fluidized-bed reactor system.

support around 300 °C,37,45 which could affect the evaluation of the amine loading. However, all samples were tested according to the same procedure in the TGA, so the evaluated results should still reflect the trend of the change in amine loading. Dual-Fluidized-Bed System Description. As shown in Figure 2, the dual-fluidized-bed system included the absorber (labeled 1), which consisted of a bubbling bed; the riser (2), consisting of a fast fluidized bed as the transport line; a cyclone separator (3) to recycle the solid material and release gas through the gas outlet; upper and lower loop seals (4, 6) to transport the material and control the solids circulation rate; and the regenerator (5), which was also a bubbling bed. Bubbling fluidized beds were selected as the absorber and regenerator to improve the gas−solid contact and thereby increase the reaction efficiency and regeneration extent. The exhaust gas from the regenerator and cyclone was introduced into a water vessel (7) to dissolve the amine vapor caused by the evaporation. The CO2 concentration leaving the cyclone was monitored by a gas analyzer. The temperatures of the regenerator, the lower loop seal, and the absorber were measured by thermocouples. The system was operated at atmospheric pressure, and transport lines were used to connect the different sections. The absorber was a circular column with an inside diameter of 50 mm and a height of 260 mm that was cooled by circulating cold water. The riser was also a circular column of 20-mm inner diameter and 1720-mm height. The cylinder section of the cyclone was 100 mm in inner diameter and 345 mm in height, with a gas outlet tube of 70-mm inner diameter. The loop seals were both rectangular with a cross

In the experiments, the temperature of the sorbents and liquid TEPA was kept at 140 °C. Here, only thermal stability was investigated, so pure N2 was introduced into the reactor to inhibit CO2-induced degradation. As shown in Figure 1, the nitrogen input was split into two parts, labeled gas 1 and gas 2. Only gas 2 was introduced through liquid TEPA and then recombined with gas 1. First, a test was run with no liquid TEPA in the lower stage to study the effect of the fluidized-bed reactor on the thermal stability of the sorbent. Pure N2 was introduced at a flow rate of approximately 600 mL·min−1. Second, about 10 mL of liquid TEPA was placed in the lower stage to study the effect of the concentration of amine vapor on the thermal stability. Because no data were available on the saturated vapor pressure of TEPA at 140 °C, the concentration of amine vapor was controlled and adjusted through the flow rates of gas 1 and gas 2. At first, the flow rates of gas 1 and gas 2 were both 300 mL·min−1 at a total flow rate of 600 mL·min−1. Then, all of the N2 was introduced through gas 2 at a flow rate of 600 mL·min−1, and gas 1 was not used. About 0.1 g of sorbent was sampled from the reactor every 3 or 5 h to analyze the adsorption capacity and amine loading in the TGA. For the TGA experiments, the sorbent was treated for 30 min under pure nitrogen at a flow rate of 100 mL·min−1 at 90 °C to remove the moisture, and then it was allowed to adsorb CO2 under 10 vol % CO2 at 60 °C to measure the adsorption capacity. After that, the temperature was increased to 300 °C under pure nitrogen, so that the sorbent would lose all of the amine impregnated. According to the percentage of amine lost, the amine loading could be calculated. It should be noted that amine groups can pyrolyze and leave carbon on the 2086



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section of 90 × 40 mm2 and a height of 350 mm. The lower loop seal was cooled by circulating cold water. The regenerator was also a cylindrical column with an inner diameter of 50 mm and a height of 692 mm. The regenerator was heated by circulating hot oil so that the column could be heated uniformly. Nitrogen was input as the fluidization gas in the regenerator so that the sorbent could be regenerated at lower temperature (∼110 °C). Secondary nitrogen was introduced at the bottom of the riser. Therefore, the stream of CO2/N2 mixture plus the secondary nitrogen transported the supported amine particles into the cyclone and the upper loop seal. Then, the particles flew into the regenerator under the driving force of the pressure drop between the two sides of the upper loop seal. The supported amine released CO2 in the regenerator and regained the capacity to adsorb CO2. Then, the sorbent flew into the absorber through the lower loop seal. The gas from the absorber and the regenerator was introduced into a bubble vessel filled with water to remove possible evaporated amine before being emitted into the atmosphere. The exhaust gas from the cyclone outlet was sampled by an online gas analyzer to monitor the CO2 concentration and estimate the CO2 capture efficiency. The pressure in the cyclone should be higher than that in the regenerator so that the gas released in the regenerator cannot enter the cyclone. Therefore, most of the gas introduced into the upper loop seal would escape through the regenerator exhaust gas outlet. The flow rate of fluidization gas introduced into each section under steady operation is listed in Table 1.

For supported amine sorbents, potential regeneration approaches include pressure-swing and temperature-swing processes. A pressure-swing process is not applicable for flue gas because of the low CO2 concentration and large amount of flue gas. Temperature-swing processes have mostly been employed in the literature to provide two driving forces for regeneration: (1) a partial pressure driving force by passing inert gas flow over the samples and (2) heat input to break the amine−CO2 bonds. To obtain high-purity CO2, two potential practical regeneration methods are (1) heating the sorbent in a pure CO2 stream30 and (2) applying steam stripping.44 The steam stripping method can increase the energy consumption of the whole system because of the production of steam. Heating the sorbent in a pure CO2 stream is the ideal regeneration approach for CCS. However, the theoretical regeneration temperature increases with increasing CO2 concentration in the regenerator for thermal regeneration under atmospheric pressure, resulting in the loss of amine groups. For wet-impregnated solid amine sorbents, the amine molecules adhere to the porous support surface by physical van der Waals forces or hydrogen bonding. At high temperature, liquid amine molecules will overcome the weak van der Waals forces or hydrogen bonding and diffuse into the bulk gas stream because of the difference in concentration, which will contribute to the evaporation of amine groups. The molecular weight of the amines will affect the strength of the interactions between the amine compounds and the support. Therefore, the thermal stability of an amine compound is related to the molecular weight of the amine, as shown in Figure 3.

Table 1. Atmosphere Introduced into a Dual-Fluidized-Bed Reactor section absorber riser upper loop seal lower loop seal regenerator

fluidization gas

flow rate (L·h−1)

CO2 N2 N2 N2 N2 N2

60 540 420 220 140 225

The CO2 capture efficiency was found to depend mainly on the sorbent circulation rate and working capacity, which was affected by two dominant factors, the regeneration temperature and the contact time in the absorber. For the dual-fluidized-bed experiments, the regeneration temperature was crucial and directly determined the regeneration degree. In the regenerator, there was some percentage of CO2 due to the regeneration of sorbent, resulting in a higher regeneration temperature even though pure nitrogen was purged. However, the evaporation of amine groups would get worse at higher temperature, and some CO2-induced degradation due to urea linkage formation would occur above 135 °C,30 which would reduce the lifetime of the sorbent. After the treatment in the dual-fluidized-bed system, some sorbent samples were analyzed by TGA to measure the change in adsorption capacity and amine loading. For these TGA experiments, the sorbent was treated according to the process mentioned above.

Figure 3. Effect of amine molecular weight on the sorbent thermal stability with temperature increasing from 100 to 400 °C.

For TEPA loaded on Q-10 silica gel, the sample started to lose weight as a result of amine evaporation at approximately 110 °C and reached the maximum loss rate of ∼2.2 wt %·min−1 at 200 °C. With a further increase in temperature, the rate decreased because few amine groups were left. The PEI-based sorbents displayed good thermal stability below 150 °C, after which the amine loss rate increased with temperature. In addition, the PEI-functionalized sorbents showed better stability, as the peak of the maximum weight change rate moved to higher temperature. Under pure N2, an increase in temperature above 270 °C led to the decomposition of surface NH2 groups in the form of NH3.37,45 Therefore, for PEI-based sorbents, a common peak around 350 °C appeared because of

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RESULTS AND DISCUSSION Thermal Stability and Cyclic Reactivity of Amines of Different Molecular Weights. In a practical process of CO2 capture, especially for CCS, a high purity of CO2 is required. 2087



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8.7%, which indicates that the TEPA-based sorbent was not sufficiently stable at 105 °C for prolonged CO2 capture. For the PEI-based sorbents, the adsorption capacity remained almost constant. The adsorption capacity decreased with increasing amine molecular weight, being approximately 2.5, 2.2, 1.7, and 1.6 mmol·g−1 for TEPA/Q-10, PEI600/Q-10, PEI1200/Q-10, and PEI1800/Q-10, respectively. TEPA has a higher amine content (26.4 mmol·g−1) than PEI (23.2 mmol·g−1), and approximately 70% of the amine groups in PEI can react with CO2 directly, which contributes to the higher adsorption capacity of TEPA-based sorbents. Figure 6 shows the weight reduction percentage of the sorbents after every cycle, which was ascribed to the

the decomposition and destruction of amine groups. For PEI1200 and PEI1800, there was another peak around 290 °C, but PEI600 had just one peak around 350 °C, indicating that the decompositions of PEI1200 and PEI1800 occurred in two steps. PEI1200 and PEI1800 might decompose into lowermolecular-weight amine groups around 290 °C, with all of the amine groups then decomposing and turning into NH3 around 350 °C. Consequently, amine evaporation can be inhibited greatly by employing high-molecular-weight amine-based sorbents. In addition, if the supported amine is regenerated under 80 vol % CO2 atmosphere, the thermal regeneration temperature should be increased above 135 °C.42 Figure 4 depicts thermal

Figure 6. Weight losses of amines of different molecular weights for each cycle in a TGA.

Figure 4. Thermal stability with time at 140 °C of supported amine sorbents of different molecular weights.

evaporation of amine. After 10 cycles, the weights of the four types of sorbent decreased by about 5.1%, 0.6%, 0.2%, and 0.1%, respectively, with increasing molecular weight. Thermal Stability in a Single-Fluidized-Bed Reactor. To obtain a high-purity CO2 stream in the regenerator, the theoretical thermal regeneration temperature should be higher than 135 °C,42 and under these conditions, the issue of thermal stability is significant and has not yet been reported. The thermal stability of the supported amine sorbent TEPA/Q-10 was studied in a fluidized-bed reactor at 140 °C under pure nitrogen. The changes in the amine loading of TEPA/Q-10 in a TGA and in a single-fluidized-bed reactor are shown in Figure 7. For the TGA experiments under pure nitrogen at 140 °C, the amine groups were lost rapidly because of evaporation, with almost all of the amine groups lost in 7 h. In contrast, in the fluidized-bed reactor, the loading remained stable for 10 h and decreased from 23% to 18% in 25 h. In the first 7 h, a slight increase in amine loading occurred because the amine lost by the sorbent at the bottom of bed entered the pores of the sorbent at the top of bed and increased the amine loading. Samples of ∼0.1 g were sampled from the top of the bed and analyzed by TGA. After 57 h, the amine loading was about 7% remaining, so that about 70% of the amine groups had been lost. Correspondingly, Figure 8 demonstrates that the adsorption capacity decreased with time in the singlefluidized-bed reactor because of the amine loss. The adsorption capacity was initially 2.6 mmol·g−1 and dropped to 0.7 mmol·g−1 after 57 h. These results indicate that the thermal stability was greatly improved in the fluidized bed because some percentage of amine was present in the gas phase in the

stability with time at 140 °C of the sorbents investigated in this work. TEPA was lost completely in 5 h, whereas only 2.5 wt % of the PEI600 was lost. Comparatively, PEI1200 and PEI1800 appeared to be very stable for 5 h. Hence, high-molecularweight amines should be employed to inhibit evaporation. The adsorption capacities of four types of supported amine in 10 cycles are displayed in Figure 5. For TEPA/Q-10, the CO2 adsorption capacity decreased continuously after each cycle from 2.53 to 2.31 mmol·g−1, for a loss of approximately

Figure 5. Cyclic adsorption capacities of amines of different molecular weights. 2088



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where R is the universal gas constant. In the TGA experiments, only several milligrams of material was used, and the evaporating amine vapor was much less than the gas stream input. Therefore, in the TGA, there was almost no amine vapor in the gas phase; that is, Ci,∞ = 0. In the fluidized-bed reactor, the sorbent amount was much greater, and the gas stream resided in the reactor for a certain time. The evaporating amine vapor remained in the reactor for some time, which implies that the amine vapor concentration in gas phase was positive; that is, Ci,∞ >0. Accordingly, the volatilization of supported amine was obviously inhibited in the fluidized-bed reactor. As mentioned previously, the amine vapor in the gas phase inhibits the evaporation of amine. Therefore, a novel strategy is proposed here to solve the problem of amine evaporation. As seen in Figure 1, gas 2 was introduced through liquid TEPA, which would increase the amine vapor concentration in the gas phase. Figures 9 and 10 show the effects of added amine vapor Figure 7. Losses of the supported amine under pure N2 at 140 °C in a single-fluidized-bed reactor and in a TGA.

Figure 9. Effect of amine vapor entrained in the reactor on the loss of the supported amine in a single-fluidized-bed reactor under pure N2 at 140 °C.

Figure 8. CO2 adsorption capacity with time in a single-fluidized-bed reactor.

on the thermal stability compared with the absence of entrained amine vapor in the single-fluidized-bed reactor. Gas 2 was input at a flow rate of 300 mL·min−1 starting at 12 h, and the trend of

fluidization gas stream, which inhibited amine evaporation because the diffusion was reduced. The amine lost stayed in the regeneration atmosphere to protect the supported amine from evaporating. The rate of amine vaporization in this system is governed by gradient diffusion, with the flux of amine vapor into the gas phase related to the gradient of the amine vapor concentration between the sorbent surface and the bulk gas Ni = kc(Ci ,s − Ci , ∞)

(1)

where Ni is the molar flux of amine vapor (mol·m−2·s−1), kc is the mass-transfer coefficient (m·s−1), Ci,s is the amine vapor concentration at the sorbent surface (mol·m−3), and Ci,∞ is the amine vapor concentration in the bulk gas (mol·m−3). The concentration of amine vapor at the sorbent surface was evaluated by assuming that the partial pressure of amine vapor at the interface was equal to the saturated amine vapor pressure, Psat, at the regeneration temperature, Treg Ci,s =

psat (Treg) RTreg

Figure 10. Change in adsorption capacity with amine vapor entrained in a single-fluidized-bed reactor under pure N2 at 140 °C.

(2) 2089



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efficiency of more than 80%, the regenerator temperature should be increased above 120 °C. In fact, in the regenerator, the temperature is greatly restricted by the solids circulation rate. High solid circulation rates result in significant fluctuations in temperature for the intense mixing of cold cycling material and hot material in the regenerator. In addition, a great deal of heat would be needed for the increase in temperature of the cold cycling sorbent flowing into the regenerator in a short time. Further investigations will be conducted on the temperature control of the system to obtain a higher CO2 capture efficiency and more stable operation of the system. This phenomenon can be explained with the solid sorbent material balance for the absorber according to the equation

amine loss was inhibited. The amine loading remained at approximately 25% with a corresponding adsorption capacity of 1.7 mmol·g−1. When the flow rate of gas 2 was increased to 600 mL·min−1 at 81 h, the amine loading and adsorption capacity increased. Figures 9 and 10 clearly indicate that entraining amine vapor into a fluidized-bed reactor is an effective and simple method for solving the problem of amine evaporation at high temperature. In addition, for higher-molecular-weight amines such as PEI, the thermal stability should be better in a fluidized-bed reactor compared with that of TEPA. Conclusively, the thermal stability of supported amine sorbents can be solved by the strategy proposed here. However, for the application of supported amine sorbents, the design of the reactor might be a potential challenge. Unfortunately, very few articles in the literature describe the performance of supported amine sorbents from the perspective of reactor design. Therefore, a dual-fluidized-bed reactor was designed to demonstrate continuous CO2 capture using supported amine sorbents. Continuous CO2 Capture Experiment in a DualFluidized-Bed Reactor. A dual-fluidized-bed reactor was utilized to simulate the practical process of CO2 capture using supported amine sorbents. Regarding supported amine sorbents, the crucial parameter to control is temperature because high heat-exchange efficiency would be required because of the high reaction rate. Moreover, the change in sorbent sensible heat between the regenerator and the absorber also increased the challenge of temperature control. Figure 11

mR 1 = [(kg of sorbent) ·(mol of CO2 )−1] Δc FCO2ECO2

(3)

−1

where FCO2 (mol·s ) is the total molar flow rate of CO2 in the inlet flue gas, ECO2 is the CO2 capture efficiency, mR [(kg of sorbent)·(mol of CO2)−1·s−1] is the sorbent mass flow rate from the absorber to the regenerator, and Δc [(mol of CO2)·(kg of sorbent)−1] is the working capacity. The captured CO2 amount can be expressed as FCO2ECO2 = mR Δc (mol of CO2 )

(4)

The capture efficiency can be written as ECO2 = mR Δc /FCO2

(5)

During the experiments, the solids circulation rate was kept almost the same, which means that mR in eqs 3−5 is constant. The sorbent working capacity (Δc) is a critical parameter and affects the mass and heat balance of the dual-fluidized-bed reactor system. From eq 3, it can be seen that the sorbent mass flow rate (mR/FCO2ECO2) varies as a function of the working capacity (Δc) and decreases with increasing Δc. The value of Δc requires knowledge of the extent of conversion of the amine sorbent in both the absorber and the regenerator. At low values of Δ c, a large amount of sorbent is required to absorb a fixed amount of CO2 from the flue gas; thus, a large value of mR/ FCO2ECO2 is needed. As Δc increases, a smaller amount of recycled sorbent, mR/FCO2ECO2, is required. At the same time, the capacity after regeneration (creg) also has an important effect on mR/FCO2ECO2. If it is assumed that the solid amine sorbent is completely regenerated in the regenerator, the capacity after regeneration (creg) is equal to zero, and Δc = cads (the adsorption capacity of sorbent); in this case, mR/FCO2ECO2 approaches the minimum value. In addition to the mass balance, the working capacity also has a large effect on the heat balance. In the absorber, the heat carried by the solid sorbent from the regenerator and produced by the adsorption reaction must be removed to keep a suitable absorber operating temperature. The amount of flue gas is fixed, so for a given CO2 removal efficiency, the amounts of heat to be released for adsorption (Qads,out) and required for regeneration (Qreg,in) depend on (1) the solids circulation rate between the absorber and regenerator, which, in turn, depends on Δc (i.e., cabs and creg); (2) the specific heat (heat capacity) of the sorbent; (3) the reaction heat between CO2 and amine; and (4) the latent heat of the gas. The sensible heating of the sorbent materials is important for this CO2 capture process, depending on the value of Δc. The heat to be evacuated from the absorber decreases

Figure 11. Change in outlet CO2 concentration with increasing regeneration temperature.

demonstrates that the CO2 outlet concentration changed significantly and rapidly with the regeneration temperature, which was increased from 115 to 124 °C in 2 min. Moreover, the CO2 capture efficiency increased almost linearly with the temperature. However, because the temperature was increased very rapidly and there was not enough time for most sorbent to regenerate, the obtained absolute CO2 capture efficiency did not represent the real efficiency at the corresponding temperature. Figure 11 shows that there was an obvious entrainment effect of CO2 capture efficiency with temperature. Therefore, the temperature of the regenerator is of great significance in improving CO2 capture efficiency because this temperature determines the extent of regeneration and the working capacity of the sorbent. To obtain a CO2 capture 2090



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with increasing Δc. This is related to the decrease in the solids circulation rate at higher Δc. When the solids circulation rate decreases, the heat carried by the hot solid from the regenerator is reduced, resulting in a decrease of Qads,out. According to this discussion, the sorbent capacities (cads and creg) in both the absorber and regenerator are very important for the mass and heat balances. From the TGA results, the CO2 adsorption step is fast, which indicates that the absorber might not be a limiting factor if a suitable adsorption reactor is used. From the continuous CO2 capture results in the dual-fluidizedbed-reactor system, sorbent regeneration in the regenerator and solids mixing in the absorber are the limiting factors for the low CO2 capture efficiency. Poor solids mixing in the absorber is a reactor problem and could easily be improved at a later stage. Sorbent regeneration is really a problem. Theoretically, the solid amine sorbent should be regenerated completely in the regenerator; therefore, creg = 0, and Δc = cads; in this case, mR/ FCO2ECO2, Qads,out/FCO2ECO2, and Qreg,in/FCO2ECO2 approach their minimum values. However, this might be difficult for a fluidized-bed reactor because amine sorbent regeneration depends on many factors such as temperature, CO2 partial pressure, kinetics, equilibrium characteristics, and hydrodynamics. According to the preceding analysis, continuous CO2 capture is demonstrated in Figure 12. The initial CO2 concentration at

decreased in a short time. In Figure 12, the CO2 concentration at the outlet of cyclone was reduced to the range of 1.0−1.5 vol % with a corresponding CO2 capture efficiency of 82−73%. It can also be seen that, when the regeneration temperature was about 120 °C, the CO2 outlet concentration was reduced to approximately 1.0 vol % with a corresponding CO2 capture efficiency of ∼80% and a sorbent working capacity of ∼0.7 mmol·g−1. When the regenerator temperature was decreased below 110 °C, the CO2 capture efficiency was no more than 70%, which further indicates that the CO2 capture efficiency was affected by the regeneration temperature. After the dual-fluidized-bed experiments, some samples were sampled and analyzed by TGA. According to the post-analysis results, the sorbent adsorption capacity and amine loading before and after the long-term operation were both equal to the capacity and loading of fresh sorbent, 2.1 mmol·g−1 and 33 wt %, respectively, indicating that the sorbent had perfect stability under continuous operation.

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CONCLUSIONS In this study, supported amine sorbents were prepared by impregnating silica particles with amines of different molecular weights (TEPA and PEIs). The thermal stability as a function of molecular weight and temperature was investigated in detail. At the regeneration temperature of 105 °C over 10 cycles, the CO2 capacities of four types of amine-functionalized sorbents remained stable, and the masses of sorbent decreased by 5.1%, 0.6%, 0.2%, and 0.1% with increasing molecular weight of amine, indicating that the PEI-based sorbents had excellent thermal stability. The thermal stability was also studied in a fluidized bed at 140 °C for 100 h using TEPA/Q-10. The results indicated that the thermal stability was much improved in the fluidized bed because the atmosphere in the fluidized bed contained amine vapor, thus inhibiting amine evaporation, as reported here for the first time. Therefore, a novel strategy was proposed here to solve the issue of amine evaporation by entraining some amine vapor into the reactor, which was demonstrated to be an effective and simple method. A dual-bubbling-fluidized-bed-reactor system was constructed to demonstrate the process feasibility of continuous CO2 capture from flue gas with supported amine sorbent. Long-term stable operation and continuous solids circulation between the two reactors was achieved, and the CO2 in the flue gas was continuously captured by the supported amine sorbent. The experimental results indicate that ∼80% CO2 capture efficiency could be achieved and that the sorbent exhibited very good stability.

Figure 12. Demonstration of continuous CO2 capture in a dualfluidized-bed system.

the outlet of the cyclone was approximately 5.5 vol %, which was diluted by nitrogen from the riser and loop seals. The solids circulating rate was approximately 50 g·min−1. After the material was fluidized and transported smoothly and steadily, the regenerator was heated with an oil bath, and the absorber was cooled using circulating cold water. The sorbent started to regenerate with increasing regenerator temperature and flew into the absorber to capture CO2. Therefore, the CO2 concentration exiting the cyclone decreased with time and with increasing regenerator temperature, as presented in Figure 12. The temperature of the regenerator was not stable because cold sorbent entered the regenerator from the absorber and hot sorbent exited the regenerator. The CO2 capture efficiency was very sensitive to regeneration temperature. When the regeneration temperature was increased, the regeneration extent of the sorbent improved, and the working capacity also increased. Hence, the CO2 concentration exiting the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-62789955. Fax: +86-10-62770209. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (2012AA06A115). 2091



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Investigation of Thermal Stability and Continuous CO2 Capture from

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